It is often desirable to convert raw hydrocarbon mixtures such as crude oil and other petroleum feedstocks to commercially valuable fuels. A number of processes for cracking hydrocarbons are known. These processes include fluid catalytic cracking (FCC) (including the FCC process of Ashland/UOP known as reduced crude conversion (RCC)). These processes are described in Venuto and Habib, "Fluid Catalytic Cracking with Zeolite Catalysts", Marcel Dekker, Inc., 1979 and Busch et al., "Reduced Crude Conversion--1: RCC Complex Now Cornerstone of Ashland Refinery", Oil & Gas Journal Dec. 10, 1984.
The cracking of hydrocarbons is accomplished by contacting the hydrocarbon to be cracked with a catalyst at elevated temperatures. The catalysts most commonly used for cracking hydrocarbons comprise a crystalline aluminosilicate zeolite that has been incorporated into a matrix. These zeolites are well known and have been described, for example, in U.S Pat. Nos. 4,432,890, 4,707,461 and 4,465,779.
The matrix into which the zeolite is incorporated may be natural or synthetic and, typically, has substantially less and in some cases no catalytic activity relative to the zeolite component. Some known matrices include clays, silica, metal oxides such as alumina and mixtures thereof.
A major difficulty with cracking catalysts is their tendency to become poisoned following contact with certain metal contaminants present in the hydrocarbon feedstock. The deleterious metals include vanadium, nickel, iron and copper. These metals may be present in the hydrocarbon as free metals or as components of inorganic and organic compounds such as porphyrins and asphaltenes. Poisoning leads to loss of selectivity, which causes increased amounts of undesirable products such as coke and light gases, i.e., hydrogen, methane and ethane. The deleterious effect of metals on cracking catalysts has been discussed, for example, in U.S. Pat. Nos. 4,376,696, 4,513,093, and 4,515,900.
Methods for counteracting the deleterious effects of metals have been developed. For example, it is known to treat FCC catalysts containing such metal contaminants with certain passivating gases. The passivating gases may, for example, be reducing gases or sulfur-containing gases.
Reducing gases used for passivating metals on an FCC cracking catalyst include hydrogen, carbon monoxide, and hydrocarbons. Sources of these reducing gases include, for example, hydrogen streams, cat cracker tail-gas, catalytic reformer tail-gas, spent hydrogen streams from catalytic hydroprocessing, synthesis gas, steam cracker gas, flue gas and mixtures thereof.
The efficiency of contacting the catalyst and the reducing gas has been recognized as being important. For example, the residence time required for passivation is discussed in U.S. Pat. No. 4,666,584 (see column 6, line 25 et seq.) and U.S. Pat. No. 4,522,704 (see column 6, line 32 et sec.). Separate passivation zones for contacting cracking catalysts and reducing gases are disclosed in U.S. Pat. Nos. 4,504,379 4,504,380, 4,409,093, 4280,895 and 4,522,704.
Less is known about the conditions for passivating gases with sulfur-containing compounds. U.S. Pat. No. 4,541,923 discloses that hydrogen sulfide may accompany the lift gas in an FCC process (see column 6, line 7, et sec.). Recycled water containing hydrogen sulfide is disclosed as being useful for passivating metals during hydrocarbon cracking at column 5, line 65, et seq. of U.S. Pat. No. 4,432,864.
Nevertheless, not enough is known about how to maximize the passivation of metals in an FCC process using sulfur-containing gases. For example, the effects of residence time and the point of contact between the catalyst and the gas have been insufficiently explored In fact U.S. Pat. No. 4,404,089 discloses that the point of contact is not critical (see column 3, line 57 et seq). There is a need, therefore, for improved methods for passivating metals on an FCC catalyst with sulfur-containing compounds. In particular, there is a need for improvements that maximize the efficiency of such methods.